By 2050, the American population will consist of 88.5 million older adults with a rising prevalence of frailty. Frailty identifies those with diminished physiologic reserve and is increasingly being used to recognize individuals at high risk of rehospitalization, surgical mortality, and health care resource utilization. As a geriatrician with advanced training in Epidemiology, I struggle daily to care for complex, frail patients and recognize the need for accurate tools to quantify frailty risk, trend progression, and to, importantly, guide interventions. Low self-reported physical activity participation is part of the clinical frailty syndrome, yet self- report fails to adequately identify those exhibiting this critrion in their daily lives due to inherent biases. Relying on self-report alone risks under-identifying high-risk individuals and limiting our ability to address this potentially modifiable component. Very little is known about how patterns of activity and sedentary behavior measured by objective monitoring are related to frailty-associated outcomes and how these measures can be applied clinically to predict frailty progression and to guide individualized treatment. My immediate goal and the objective of this proposal is to study how measures of (in)activity relate to frailty and aging outcomes and how they can be used to inform a frailty intervention. I hypothesize that unique, identifiable patterns of (in)activity as measured by activity monitors will predict those who go on to experience a decline in their frailty and aging measures and can be used to tailor treatment recommendations. To test these hypotheses, the following aims are proposed: Aim 1) Characterize activity and sedentary behavior among non-frail, pre-frail, and frail subgroups using activity monitors and relate these measures to 5-year frailty and aging outcomes; Aim 2) In my independent, longitudinal frailty study, relate activity, sedentary behavior, sit-to-stand transitions and cadence to change in frailty components and aging outcomes at 1-year; and Aim 3) Design and pilot test a frailty intervention tool that addresses activity barriers among frail adults and tailors activity targets for frail individuals using identified accelerometry deficits. will relate activity monitor output from the National Social Life, Health and Aging Project data (2010-2011, 2015-2016) to frailty and aging outcomes at 5 years. I will then translate these findings in my established clinical cohort (Successful Aging and Frailty Evaluation clinic) of predominantly pre-frail and frail adults using activity monitoring and 1-year outcomes. Finally, I will use the findings from these studies in combination with patient and professional stakeholder input to design a sustainable intervention appropriate for frail elders. These proposed studies will identify modifiable activity and sedentary behaviors that can be measured using activity monitoring to detect frailty risk factors, predict progression, and guide treatment, and thus advance our understanding of the role of activity and sedentary behavior in the pathophysiology and treatment of frailty. During the award period, I will receive critical mentorship from nationally-recognized aging researchers, Drs. Linda Waite and William Dale, and supplementary guidance from experts in activity (Dr. Amy Luke), sedentary behavior (Dr. David Conroy), energy expenditure (Dr. Dale Schoeller), frailty (Dr. Jeremy Walston), longitudinal survey and clinical trial analysis (Dr. Masha Kocherginsky), and interventional trials in older adults (Dr. Le Lindquist). I will obtain advanced training in frailty, activity monitoring, statistics, clinical tial design, and writing. This work will lay the foundation for my long-term goal of becoming an independent clinical investigator with expertise in the diagnosis and management of frailty.